> Some sales taxes are conditional at the point-of-sale

What does this mean, and why is it relevant to anything? I feel like you’re trying very hard to insist that taxes are impossible to calculate.

> there is no single tax-included price

There’s no single pre-tax price either. All of this taken together makes it trivial to tune the post-tax price to round to the nearest nickel even if your tax authority hypothetically insists rounding is forbidden. The POS system says you owe 31.67 based on the ticket price and the applicable tax, rounds to a target of 31.65, and then applies a 2 cent discount pretax to your purchase to make this work out.

Hell, gas stations have proven prices can be in fractional cents so you could even calculate and apply a fractional discount if you find an edge case where you can’t discount whole cents and still round to the nearest nickel.